The portion of the ice sheet that flows into the Ross Sea is thinning in
some places and thickening in others, because of a delayed response to the end
of the last global glacial cycle as well as internal instability, not because of
any current climate change. Its near-future impact on global sea level is thus
already determined regardless of climate change, but too little of the ice sheet
has been surveyed to predict its overall future behavior.

A time series of the net rate of snow accumulation since 1806, derived from
the thicknesses of annual layers in ice cores, shows an increase to about 20%
above the long-term mean following a minimum around 1960. Other data suggest
this increase is widespread over Antarctica and should contribute to a lowering
of sea level of 1.0-1.2 mm per year.

Radiocarbon dating of sediments, from three sites near ice shelves or
smaller ice tongues close to outlet glaciers, established the history of ice
advances or retreats reflected respectively in alternating layers of glacial
debris or remains of marine algae. Antarctic ice fringes have advanced during
past warm intervals, and reached their greatest extent 4,000-7,000 years ago,
when global temperatures were 1° or 2° C warmer than present.

Some glaciologists have suggested that the ice sheet, which is grounded
below sea level, could become unstable and disintegrate, but geologic evidence
of such an occurrence in the past has been lacking. This study found evidence in
sediments of ice-free conditions during certain Cenozoic intervals, lending
credence to the concern that global warming and rising sea levels could lead to
collapse of the ice sheet in the near future.